Containers and sheets made of filled polymer compositions

ABSTRACT

According to one embodiment, a container comprises a base. The base has a bottom wall and a continuous base wall which encompasses the bottom wall and extends upwardly therefrom. The base is made from a mixture of a polyolefin and a filler. The mixture is from about 30 to about 75 wt. % filler and from about 25 to about 70 wt. % polyolefin. A high aspect ratio filler and a low aspect ratio filler are included in the filler, with the high aspect ratio filler having an aspect ratio of at least 5:1 and the low aspect ratio filler having an aspect ratio of less than about 3:1. The filler comprises at least 50 wt. % of low aspect ratio filler.

FIELD OF THE INVENTION

The present invention relates generally to packaging containers andsheets and, more particularly, to mineral-filled polyolefin foodcontainers and sheets.

BACKGROUND OF THE INVENTION

The use of inexpensive plastic food packaging containers and sheets hasbecome popular for dispensing and serving various food products.Generally, these food containers and sheets are typically made of aolefin polymer, such as polypropylene, or polyethylene, or a styrenicpolymer such as polystyrene, including high impact polystyrene.

One drawback of the above materials for making plastic food packagingcontainers and sheets is the lack of sufficient flexural modulus coupledwith good impact and good heat resistance (i.e., microwaveable). Amaterial that lacks sufficient flexural modulus, polyethylene orpolypropylene for instance, will require a much greater thickness tomake the container or sheet functional from the rigidity point of view.The increase in material thickness, however, increases the cost ofmanufacturing. If a material has a low impact property, polystyrene, forinstance, its resulting container or sheet is more brittle and,therefore, more likely to break during use than a container or sheetmade with a material having a higher impact property. A sheet orcontainer made with a material having a low heat resistance will resultin the loss of modulus or stiffness at higher temperatures, making thecontainer or sheet not functional.

By adding a mineral filler to the polyolefin, rigidity can be increased.One filler widely used is talc. There are tradeoffs, however, in usingtalc as a filler. For example, the impact property of a talc-filledpolyolefin container decreases as the wt. % of talc in the container orsheet increases. As mentioned above, a container or sheet with a lowimpact is brittle and can easily fracture or shatter upon dropping. Talcloading of greater than about 50 wt. % usually results in a compositewhich is too brittle to be used in most applications. For someapplications, however, it is desirable to use more than 50 wt. % fillerbecause that decreases the cost of the container or sheet and makes thecontainer or sheet more rigid.

Accordingly, a need exists for a container or sheet, such as apolyolefin container or sheet, that has a desirable flexural moduluswhile also having a desirable impact property and heat resistance.

SUMMARY OF THE INVENTION

According to one embodiment, a container that is adapted to hold foodcomprises a base. The base has a bottom wall and a continuous base wall,which encompasses the bottom wall and extends upwardly therefrom. Thebase is made from a mixture of a polyolefin and a filler. The mixturecomprises from about 30 to about 75 wt. % filler and from about 25 toabout 70 wt. % polyolefin. A high aspect ratio filler and a low aspectratio filler are included in the filler, with the high aspect ratiofiller having an aspect ratio of at least 5:1 and the low aspect ratiofiller having an aspect ratio of less than about 3:1. The fillercomprises at least 50 wt. % of low aspect ratio filler.

In another embodiment, a container that is adapted to hold foodcomprises a base having a bottom wall and a continuous base wall. Thecontinuous base wall encompasses the bottom wall and extends upwardlytherefrom. The base is made from a mixture of a polyolefin and a filler,while the filler includes a mixture of from about 20 to about 50 wt. %talc and from about 50 to about 80 wt. % calcium carbonate.

A third embodiment has a container adapted to hold food, which comprisesa base. The base has a bottom wall and a continuous base wall. Thecontinuous base wall encompasses the bottom wall and extends upwardlytherefrom. The base is made from a mixture of a polymer and a filler,and the filler includes a mixture of from about 20 to about 50 wt. % ofa first filler and from about 50 to about 80 wt. % of a second filler.The first filler is selected from talc, mica, wollastonite, or thecombination thereof, while the second filler is selected From calciumcarbonate, barium sulfate, or the combination thereof.

According to one embodiment, a sheet that is adapted to hold or carryfood comprises a base. The base is made from a mixture of a polyolefinand a filler. The mixture comprises from about 30 to about 75 wt. %filler and from about 25 to about 70 wt. % polyolefin. A high aspectratio filler and a low aspect ratio filler are included in the filler,with the high aspect ratio filler having an aspect ratio of at least 5:1and the low aspect ratio filler having an aspect ratio of less thanabout 3:1. The filler comprises at least 50 wt. % of low aspect ratiofiller.

In yet another embodiment, a sheet that is adapted to hold or carry foodcomprises a base. The base is made from a mixture of a polyolefin and afiller. The filler includes a mixture of from about 20 to about 50 wt. %talc and from about 50 to about 80 wt. % calcium carbonate.

Another embodiment includes a sheet adapted to hold food. The sheetcomprises a base made from a mixture of a polymer and a filler. Thefiller includes a mixture of from about 20 to about 50 wt. % of a firstfiller and from about 50 to about 80 wt. % of a second filler. The firstfiller is selected from talc, mica, wollastonite, or combinationsthereof, while the second filler is selected from calcium carbonate,barium sulfate, or combinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other advantages of the invention will become apparentupon reading the following detailed description and upon reference tothe drawings.

FIG. 1 is an isometric view of a plastic food container including anoptional lid and a base according to one embodiment of the presentinvention.

While the invention is susceptible to various modifications andalternative forms, a specific embodiment has been shown by way ofexample in the drawing and will be described in detail herein. It shouldbe understood, however, that the invention is not intended to be limitedto the particular forms disclosed. Rather, the invention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

According to one embodiment of the invention shown in FIG. 1, amulti-compartment container 10 comprises an optional cover or lid 12 anda base 14. The lid 12 may be adapted to releasably engage the base 14.The base 14 and the lid 12 may be releasably latched by a latchingmechanism 15, such as that described in U.S. Pat. No. 5,758,791. It iscontemplated that the container 10 of the present invention may bereleasably latched by other known latching mechanisms, such as thatshown in U.S. Pat. No. 5,607,709.

While the container 10 is depicted in FIG. 1 as being of a generallyoval configuration, the invention herein disclosed is not intended to belimited thereto, but may take various other geometric shapes, such ascircular and polygonal shapes. Likewise, the multi-compartment foodcontainer 10 may be a single-compartment container or a container havingmore than two compartments. In some embodiments, the container may notinclude a lid, and may just have a base with side walls, such as a plateor serving tray. In another embodiment, the invention is a sheetcomprising only a base. For example, the sheet may also be a cuttingboard or placemat.

The container (or sheet) 10 of the present invention is comprised of amixture of a polyolefin and a filler. A polyolefin includes polymersbased on olefin monomers. Two examples of a polyolefin are polyethylenesand polypropylenes. The mixture comprises from about 25 to about 70 wt.% polyolefin. In another embodiment, the mixture comprises from about 35to about 65 wt. % polyolefin.

The mixture comprises from about 30 to about 75 wt. % filler and, in oneembodiment, from about 35 to about 65 wt. % filler. The filler itself isa mixture of a high aspect ratio filler and a low aspect ratio filler.The term “aspect ratio” of a particle is defined herein as a ratio of alargest dimension of the particle divided by a smallest dimension of theparticle. The aspect ratios are determined by scanning under an electronmicroscope (2,000 times magnified) and visually viewing the outsidesurfaces of the particles to determine the lengths and thicknesses ofthe particles.

A high aspect ratio filler is defined as a filler having an aspect ratioof at least about 5:1. The high aspect ratio fillers of the presentinvention generally have an aspect of from about 5:1 to about 40:1, andpreferably from about 10:1 to about 20:1. The high aspect filler may betalc, mica, wollastonite, or combinations thereof. Commerciallyavailable talc materials include JETFIL® 575, available from LuzenacAmerica of Englewood, Colo. Commercially available mica materialsinclude SUZOREX® 325-PP, available from Zemex Industrial Minerals, Inc.Commercially available wollastonite include the NYGLOS® series ofwollastonite, available from NYCO Minerals Inc. of Calgary, Alberta,Canada.

The low aspect ratio filler may be calcium carbonate, barium sulfate, orthe combination thereof Commercially available calcium carbonateincludes OMYACARB FT®, available from OMYA Inc. of Alpharetta, Ga. Oneexample of commercially available barium sulfate is BARITE 2075®,available from Polar Minerals in Mentor, Ohio. A low aspect ratio filleris defined as being a filler having an aspect ratio of from 1:1 to about3:1, preferably from 1:1 to about 2:1.

The filler mixture of the present invention comprises at least 50 wt. %low aspect ratio filler. The filler mixture may be from about 50 toabout 80 wt. % low aspect ratio filler and from about 20 to about 50 wt.% high aspect ratio filler.

A sheet to be used in forming a container may be made from the samematerials discussed above in connection with the containers. The sheetto be used in forming a container has increased flexural modulus(rigidity), impact property, and tensile modulus. The increased flexuralmodulus is determined in accordance with ASTM D790, the increased impactproperty is determined in accordance with ASTM D5420-98a (GardnerImpact), and the tensile modulus is determined in accordance with ASTMD638. The flexural modulus of a sheet of the present invention isgenerally greater than about 350,000 pounds per square inch (psi), andpreferably greater than about 400,000 psi, as determined by ASTM D790.The impact property of a sheet of the present invention is generallygreater than about four inch-pounds (in-lbs) on a sheet of approximately0.020 inches thick, preferably eight in-lbs, as determined by ASTMD5420-98a (Gardner Impact). The tensile modulus of a sheet of thepresent invention is generally greater than about 250,000 psi, andpreferably greater than about 300,000 psi, as determined by ASTM D638.

The base 14 and the optional lid 12 may be formed using conventionalthermoforming or injecting molding processes. According to one method ofmanufacturing, pellets of a polyolefin resin are melted in a twin screwextruder. Powders of the high aspect ratio filler and low aspect ratiofiller are then added into the polyolefin melt to form a blend. Theblend is extruded through a die to form an extruded sheet. The extrudedsheet is then thermoformed to a desired shape of a container.

The thickness of the container may vary, but is typically from about0.005 to about 0.250 inch A preferable thickness is from about 0.010 toabout 0.040 inch. The base 14 may be the natural color of thepolyolefin/filler mixture, or a variety of colors or color combinations.The optional lid 12 may either be transparent, opaque, or a variety ofcolors. The height and shape of the lid 12 and/or the base 14 may varyfrom that shown without departing from the scope of the invention. Thesheet used for serving, as a placemat or as a cutting board, may have athickness from about 0.005 to about 0.040 inch. The sheet may be opaqueor a variety of colors or color combinations.

EXAMPLES

The following examples are presented to demonstrate the flexuralmodulus, the tensile modulus, and the Gardner Impact of various polymerfilled sheets. Examples 1-21 represent tests of seven different ratiosof High Aspect Ratio (HAR) filler to Low Aspect Ratio (LAR) filler. Eachratio was tested at three different weight percents of filler. Theweight percent of filler varied in each test from about 35% to about62%. The exact ratios and weight percents are shown in Table 1.

In each of the examples in Table 1, an HAR filler and an LAR filler wereblended in the specified ratio with a drum tumbler to form a fillermixture. To create a filled polymer sheet, the polymer was fed at acontrolled rate into the feed throat of a 34-mm co-rotating twin screwextruder, melted, and conveyed toward the melt pump at the end of theextruder. The filler mixture was then added to the melted polymerthrough a side-stuffer at zone 4 of a 9-zone twin screw extruder. Thepolymer/filler mixture was further melt-blended and extruded through aflat die into a sheet form of approximately 0.020 inches in thickness.Actual weight % filler was determined by performing an ash test or bycalculation from the densities of the composite, the polymer, and thefiller mixture with the following equation.Wt. % filler=[d _(f)×(d _(c) −d _(p))]/[d _(c)×(d _(f) −d_(p))]×100  Eq. (1)

-   -   where d_(f)=density of the filler    -   d_(c)=density of the composite    -   d_(p)=density of the polymer

Flexural moduli were determined from these sheets in accordance withASTM D 790. Five specimens of each sample were tested in both theMachine Direction (MD) and the Transverse Direction (TD) and thenaveraged. Tensile moduli were determined from these sheets in accordancewith ASTM D638. Five specimens cut in only the Machine Direction (MD)were tested and then averaged. Gardner Impact Mean-Failure Energy (MFE)was determined from these sheets in accordance with ASTM D5420-98a.

TABLE 1 Mechanical Properties Of Mineral Filled Polypropylene¹ TotalHAR³/ Flexural Tensile Gardner Filler LAR⁴ Modulus Modulus ImpactExample Level² (%) ratio (Kpsi) (Kpsi) MFE (in-lbs) Example 1 41.3 100/0555 410 3.0 Example 2 50.7 100/0 703 540 1.6 Example 3 60.1 100/0 871619 0.4 Example 4 44.8  75/25 515 398 5.8 Example 5 52.6  75/25 630 4464.4 Example 6 60.5  75/25 768 586 3.3 Example 7 42.3  60/40 489 359 5.9Example 8 54.4  60/40 669 517 4.3 Example 9 61.5  60/40 741 557 2.4Example 10 40.9  50/50 448 316 9.2 Example 11 45.5  50/50 488 328 9.7Example 12 62.0  50/50 748 468 8.6 Example 13 40.1  40/60 407 301 11.0Example 14 50.6  40/60 483 355 10.8 Example 15 58.6  40/60 591 420 12.3Example 16 37.8  25/75 361 263 13.6 Example 17 49.4  25/75 443 278 14.6Example 18 54.2  25/75 520 305 21.2 Example 19 37.6  0/100 279 206 21.8Example 20 51.8  0/100 314 248 22.6 Example 21 59.6  0/100 323 359 34.6¹PP homopolymer, melt flow rate (MFR) = 0.8. ²Filler level calculatedbased on Equation (1). ³High Aspect Ratio filler used was LuzenacJetFil ® 575 talc. ⁴Low Aspect Ratio filler used was OMYACarb ® FTcalcium carbonate.

As shown in Table 1, for all the HAR/LAR ratios, as the total fillerlevel increased, the flexural modulus and tensile modulus alsoincreased. This is also beneficial since filler is less expensive thanpolyolefin and, therefore, the cost of manufacturing lessens Also, it isshown that the higher the ratio of high impact ratio filler compared tolow impact ratio filler, the higher the flexural modulus and tensilemodulus of the tested sheets.

For Examples 1-9, the sheets have Gardner impact values that arerelatively small and decreased as the amount of filler increased. As thelow aspect ratio filler level increased to about 50% and above, however,the Gardner impact values surprisingly generally increased as the amountof filler increased. This is clearly shown in Examples 15 (as comparedto Examples 13 and 14) and 18 (as compared to Examples 16 and 17).Sheets having a low aspect ratio filler of less than about 40 wt. % ofthe filler (Examples 1-9) produced a Gardner impact value that decreasedas the amount of filler increased (see, e.g., Examples 4-6), resultingin a sheet that was less desirable. Since filler is currently lessexpensive to manufacture than polyolefin, it is desirable to use agreater percentage of filler in the sheets. The Gardner impact values ofExamples 1-9 were also not desirable due to their low values. A lowGardener impact value indicates a sheet that is more brittle and morelikely to break during use.

Examples 10-18 (at low aspect ratio filler levels of about 50% andabove) generally had Gardner impact values that surprisingly increasedas the total filler level increased. Examples 10-18 generally providedcompositions with a desirable combination of properties (flexuralmodulus, tensile modulus, and Gardner impact). The compositions ofExamples 10-18 were also more economical at this time as compared tounfilled polyolefins because of the cost of filler as compared topolyolefins.

Examples 19-21 (without a high aspect ratio filler) had desirableGardner impact values that also increased as the weight percent offiller increased. Examples 19-21, however, had lower flexural modulusand tensile modulus than is generally desired.

While the present invention has been described with reference to one ormore particular embodiments, those skilled in the art will recognizethat many changes may be made thereto without departing from the spiritand scope of the present invention. Each of these embodiments andobvious variations thereof is contemplated as falling within the spiritand scope of the claimed invention, which is set forth in the followingclaims.

1. A container comprising a base having a bottom wall and a continuousbase wall, the continuous base wall encompassing the bottom wall andextending upwardly therefrom, the base comprising from about 35 to about65 wt. % filler and from about 35 to about 65 wt. % polyolefin, thefiller includes talc and calcium carbonate, the filler comprising fromabout 50 wt. % to about 75 wt. % calcium carbonate, furthercharacterized by having Gardner Impact Mean Failure Energy of greaterthan about 8 in-lbs, as measured on a 0.020 inch thick sheet inaccordance with ASTM D5420-98a.
 2. The container of claim 1, wherein thecontainer further comprises a lid, the lid having a top wall and acontinuous lid wall, the lid wall encompassing the top wall andextending downwardly therefrom, the lid comprising a polyolefin and afiller, the lid comprising from about 35 to about 65 wt. % filler andfrom about 35 to about 65 wt. % polyolefin, the filler including talcand calcium carbonate, the talc having an aspect ratio of at least about5:1 and the calcium carbonate having an aspect ratio of less than about3:1, the filler comprising from about 50 wt. % to about 75 wt. % calciumcarbonate.
 3. The container of claim 1, wherein the talc has an aspectratio of from about 5:1 to about 40:1.
 4. The container of claim 3,wherein the talc has an aspect ratio of from about 10:1 to about 20:1.5. The container of claim 1, wherein the calcium carbonate has an aspectratio of from 1:1 to about 2:1.
 6. The container of claim 1, wherein thepolyolefin is a polypropylene, a polyethylene, or combinations thereof.7. The container of claim 6, wherein the polyolefin is a polypropylene.8. The container of claim 7, wherein the polyolefin is a polypropylenehomopolymer.
 9. The container of claim 7, wherein the polyolefin is animpact copolymer polypropylene.